Hydraulic systems exist having pumps that are driven by compressed air driven motors. The peak hydraulic pressures generated by the pump are generally significantly greater than the pneumatic pressure used to drive the pump, but in many applications the peak hydraulic pressure obtainable is limited by the pneumatic pressure used to drive the pump and the operation of the pump relies on the pump stalling when that pressure is reached. In some of these pumps, the pneumatic motor will never stall due to leakage, losses, system design or pressure variation in the system. Particularly for these kinds of applications, some other means is desirable to cause the pneumatic motor to stop at a given limit of hydraulic pressure.